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Q. My parents asked me to start paying for my share of our family wireless plan. Should I get my own before heading back to school?

A. Going on your own can save real money, but you may cost your parents some at the same time. That’s because the shared plans offered by all four nationwide wireless carriers — along with many “mobile virtual network operator” firms reselling their networks — really function as customer-retention tools.

That is, while they offer substantial savings for putting multiple lines on one bill, having so many services on one account complicates taking some of that business elsewhere. And subtracting a line from a shared plan will increase cost for people left on it.

Consider what unlimited (really, unmetered) data plans cost per line (including discounts for automatic payments) at the big four carriers:

  • At AT&T, you pay $46.25 a line for four lines, $55 a line for three, and $72.50 for two.
  • Sprint’s current promotion has you paying $22.50 for each of four lines, $30 for each of three and $45 for each of two.
  • T-Mobile charges $40 a line for four, $47 apiece for three lines, and $50 each for two. All cases come with only standard-definition streaming video and mobile hotspot use capped at 3G speeds.
  • Verizon Wireless asks $45 a line for four lines, $54 each for three lines, and $70 a pop for two.

Taking yourself off your parents’ plan may liberate them to switch to a cheaper plan. If they’re okay with paying a little more for not having to worry about your own use, you can start looking for cheaper options on your own.

Related: When our smartphones leave us out of touch

In a WiFi-dense environment like a college campus, the biggest savings will come from a WiFi-first service that routes calls and texts over that wireless network instead of the cellular service (usually, AT&T or T-Mobile) it resells.

One service that topped recent readers-choice surveys at Consumer Reports and PCMag was Project Fi, the service Google launched in 2015. Unlimited calling and texting via Sprint, T-Mobile and U.S. Cellular costs $20 a month, after which data runs $10 a gigabyte (with whatever fraction of a…